Xerographic liquid development



Mam}! 1963 R. w. GUNDLACH ETAL 3,

XEROGRAPHIC LIQUID DEVELOPMENT 2 Sheets-Sheet 1 Filed May 15, 1964 FIG] ' YSTEPPING MOTOR AMP INVENTO -S ROBERT w. sun

DLACH STINI BY PETER P.

VA HTORNBVS March 1968 R. w. GUNDLACH ETAL 3,372,027

XEROGRAPHIC LIQUID DEVELOPMENT Filed May 15, 1964 2 Sheets-Sheet z INVENTORS ROBERT W. GUNDLACH BY PETER P. AUGOSTINI United States Patent Ofifice 3,372,027 Patented Mar. 5, 1968 3,372,027 XEROGRAPHIC LIQUID DEVELOPMENT Robert W. Gundlach, Victor, and Peter P. Augostini,

Webster, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed May 15, 1964, Ser. No. 367,776 10 Claims. (Cl. 961) ABSTRACT OF THE DIILOSURE Latent electrostatic images are developed by applying a homogeneous liquidhaving a bulk resistivity in the rang of 5x10 ohms-ems. to ohms-ems. to the edge of a non-porous blade and wiping the edge of the blade across the latent electrostatic images.

This invention relates to xerography and in particular, to xerographic liquid development.

While xerography or electrophotography is best known as a dry process, considerable research effort has been expended on liquid developers for latent electrostatic images. The advantages that have been thought to be obtainable with liquids include higher speed, greater uniformity, higher resolution, greater ease in handling the developer, and self fixing. Electrostatic control in the application of a liquid has been well known for some time in electrostatic paint sprayers.

An early xerographic use of liquid developer is disclosed in U.S. Patent 2,551,582 in which a solvent mist is presented to the latent electrostatic image Where it is attracted selectively to the charged areas. The solvent image formed in this way can then be transferred-t0 dye transfer paper which in turn is run against a web carrying a soluble dye such as used, for example, in spirit duplicating. Perhaps the most common xerographic liquid developer is analogous to the usual powder development arrangement having a carrier fluid with a strong triboelectric effect on a particulate material suspended in the fluid. For example, a pigment is suspended in petroleum solvent and then flowed over the latent electrostatic image so that the pigment deposits out in the image areas by virtue of a triboelectric charge picked up through con tact with the carrier liquid. This is disclosed, for example, in US. Patent 3,010,842. A third type of liquid development is electrolytic in nature and development is produced by metallic ions depositing out of the electrolyte in accordance with the image. An example of this is disclosed in US. Patent 3,057,787. A fourth method might be called gravure development in which a homogeneous conductive'liquid is carried in the recesses of a surface composed of lands and recesses. This surface is brought against a latent electrotatic image and the charged image areas draw the conductive liquid out of the recesses to develop the image. This is disclosed in U.S. Patent 3,084,043.

Of these four prior art methods, only the first and last produce selective wetting action of the image carrying surface. The other two completely wet the surface and then deposit something out of the liquid onto the surface. The first method is not exactly a liquid developer since it develops with a vapor or mist. The last method, gravure development, differs from the present invention in the resistivity requirement of the developer liquid, the characteristics of the developer applicator and in the speed of operation as will become more apparent in the following disclosure. The present invention is believed to be an advance over the prior art in its greater simplicity and high speed of operation.

In accordance with the present invention, a non-porous applicator carrying developer liquid is drawn across the latent electrostatic image carrying surface so that transfer of the liquid to the surface occurs only in image areas. Some of the developer liquids suitable for this use have been found valuable in changing the surface characteristics of the latent image bearing surface so as to enable use as a duplicating master or to allow image intensification and the like. Thus, it is an object of the present invention to define a novel method xerographic liquid development.

It is also an object to define a novel apparatus for liquid development of xerographic images.

It is a further object to define a novel method of forming an offset master for water based inks.

Further objects and features of the invention will become apparent while reading the following description in connection with the drawings, wherein:

FIGURE 1 is a diagrammatic illustration of liquid development.

FIGURE 2 is a diagrammatic illustration of xerographic apparatus for high speed image reproduction.

FIGURE 3 is a second embodiment of a liquid development.

It is not truly surprising that a liquid which will normally wet a surface can be drawn across that surface in such a way that none of it will transfer to the surface. However, it has now been found that a latent electrostatic image can selectively effect transfer under the same conditions. This has given rise to the novel developing techniques disclosed below.

FIGURE 1 illustrates diagrammatically the operation of the present invention in its simplest form. A xerographic plate 10 having a photoconductive insulating layer 11 such as vitreous selenium and a conductive substrate 12 such as aluminum is electrostatically charged and exposed to an image pattern in a conventional manner to provide a latent electrostatic image indicated in FIGURE 1 by plus signs 13. An applicator 15 transports a liquid developer across the image bearing surface of plate 10. This applicator is depicted as having a handle 16 of noncritical material such as plastic, wood or metal and a blade 17 of a hard, smooth, nonporous material such as metal, glass or plastic. This blade is as wide as the image area to be developed and thin enough to be somewhat flexible to prevent scratching the plate. Thus, a thickness of about 2 mils to 10 mils is appropriate, depending upon the characteristics of the material used. For example, steel would necessarily be quite thin whereas a plastic such as mylar is preferably at least 4 mils thick. A material that is too stiff or sharp is likely to deteriorate the image bearing surface, while a material that is porous, absorbent or too soft, will spread the developer uniformly over the surface rather than selectively. The developer liquid is carried in a reservoir 18 into which applicator 15 is dipped before traversing the image bearing suface.

Wttting agent is used herein to denote a liquid synthetic substance that promotes spreading of a liquid on a surface or penetration into a material in such a way that the wetting liquid is no longer repelled.

The developer is preferably a homogeneous liquid, colored or uncolored depending upon the particular use desired, having a bulk resistivity of at least 5 X 10 ohms per cm. When a conductive applicator blade is used such as a metal applicator blade, a higher resistivity such as l0 l0 ohms per cm. is preferred for the liquid developer. When an insulating applicator blade is used such as a glass blade, a lower resistivity developer is preferred of between 5x10 ohms per cm. and 10 ohms per cm. With lower resistivities, the development speed becomes more critical since low resistivity developers tend to discharge the latent image. Developers having a resistivity below 5x10 ohms per cm. discharge the latent image quickly and then lose adhesion to the image bearing surface.

Viscosity and surface tension of the developer are not critical but should be adjusted to permit enough of the developer to cling to the applicator blade and provide flow characteristics adequate to insure a ready flow from the blade to the latent electrostatic image. It is also important that the liquid used be able to wet both the blade and the surface carrying the latent electrostatic image. The materials that have worked well both with vitreous selenium plates and binder type electrophotographic papers include wetting agents such as alkylphenyl polyglycol ether, various synthetic (non-ionic) deter-ments receptive to water, and petroleum products such as mineral oil, lubricating oil and paraffin oi. Silione oils have als been found to work quite well. Many of the synthetic detergents and wetting agents are readily colored with alcohol or water soluble dyes such as crystal violet without excessively increasing the conductivity of the liquid. The operation of the simplified device shown in FIGURE 1 is by dipping the blade 17 into the developer reservoir 18, shaking or wiping off any excessive developer, then placing the edge of the blade against one edge of the image bearing surface and drawing the blade across the surface at a rate of at least 2" per second. A lower rate of speed tends to leave background deposition. Low viscosity inks have given effective development up to 40" per second. As illustrated in the figure, some of the developer migrates from the blade onto the latent image bearing surface in charged areas but no deposition occurs in uncharged areas.

In order to insure good operation the voltage of the latent electrostatic image with respect to reference should be in the range of 300 to 1000 volts. Lower voltages produce nonuniformity and low density while higher voltages are more expensive to achieve and sometimes produce image-disruptive discharges. The reference for the latent electrostatic image is taken as the substrate of the xerographic plate. It is desirable to maintain the ink applicator at this reference voltage also, although good images have been produced without any specific connection for this purpose.

FIGURE 2 illustrates a rotary apparatus for forming images in accordance with the invention. This apparatus is comprised of a xerographic drum 20, driven by a motor 21 through operating stations positioned sequentially around the drum. Thus, there is a first cleaning station 23, then a charging station 25 followed by an exposure station 26, a developing station 27, and last, a transfer station 28. The developer station comprises a liquid developer reservoir 30, a plurality of developer applicator blades 31 mounted on a single axle for revolution through the developer reservoir and against xerographic drum 20. The axle carrying the applicator blades is rotated by a stepping motor 32 to bring a new applicator blade in contact with the drum for each frame of exposure. At exposure station 26, a photocell 33 detects the frame separations in a continuous transparency being reproduced. The photocell output is amplified by amplifier 34 to operate stepping motor 32 bringing a new applicator blade into operating position at a time When there is a separation between the images being reproduced on drum 200. The necessary delay between the time of exposure at exposure station 26 and the time of development is compensated for by positioning photocell 33 at a distance, along the transparency being reproduced, sufficiently displaced from the exposure position to provide the necessary time lag.

In operation, xerographic drum is rotated by motor 21 first through a charging station where the drum is sensitized by a corona discharge device. The drum then rotates through an exposure station depicted here as using a microfilm projector 35 which exposes the drum 2t by slit projection techniques to a microfilm transparency magnified to original document size. This exposure selectively dissipates the charge placed on the drum by sensitizing station 25 so that a latent electrostatic image is formed. This latent electrostatic image is then rotated past an applicator blade 31 carrying a film of developer liquid 36. The drum is rotated by a motor 21 carrying the latent electrostatic image past blade 31 so that the developer liquid deposits in the charged areas. The image thus developed on drum 2d is then transferred to a tranfer web 37. The corona discharge device 38 improves the transfer by placing an electrostatic charge against the backside of the transfer web while the photoconductor is illuminated through the transfer web by room light. However, it should be understood that a straight pressure transfer is possible since the developer is a liquid. "When using a Wetting agent as a developer such as alkylphenyl polyglycol ether, the developer itself may be colorless and the transfer web 37 can then be a coated paper or other water repellent material that becomes hydrophylic in areas of image transfer. The image is then developed into a visible image by application of a water base ink from roller 40. Before a new image is formed, drum 20 is then cleaned at a cleaning station 23. Where a liquid synthetic detergent is used on vitreous selenium, a sufficient application of plain water on it is usually adequate to remove the wetting agent completely.

The invention as described in relation to FIGURES l and 2 operates using a previously formed latent electrostatic image on the xerographic plate. It is also possible to develop the image simultaneously with formation. This is illustrated in FIGURE 3 using a xerographic NESA plate 45. Plate 45 is conventionally comprised of a glass substrate 46 coated with a conductive transparent coating of tin oxide 47 and then with a layer of vitreous selenium 48. This provides a xerographic plate which may be exposed through the substrate. The image to be reproduced illuminates plate 45 through the substrate 46 from a projector 50. The illumination pattern is depicted by showing cross-hatched areas indicative of where there is. little or no light reaching the plate. A potential source 51 in the range of about 300 to 1500 volts is applied between blade 17 and the transparent conductive coating 47. As blade 17 carrying the homogeneous developing liquid from reservoir 18 is wiped across the surface of plate 45, the developer liquid wets the plate preferentially in the illuminated areas producing a developed image. The ex:

posure of plate 45 as illustrated in FIGURE 3 need not bea full illumination of the plate but can also be a slit illumination in which the slit moves synchronously with the developing blade 17 exposing the photoconductor line by line at the point where blade 17 contacts the surface. In the embodiment ilustrated in FIGURE 3, the dark conductivity of the xerographic plate may be much higher than is required where a latent image must be stored on the plate for a finite period of time. Thus, a much wider range of photoconductive rntaerials may be used including many with higher photosensitivity such as cadmium. sulfide binder materials with a photoconductor-to-binder ratio such as to give higher photosensitivity than is normally achievable in a xerographic plate.

While not to be considered limiting, it is believed that the operation of the present invention may be explained as follows: The operation of fountain pens and similar devices is due tocapillary effects. The orifice of a capillarycontaining ink is brought into contact with the Writing surface and relative motion between the two carries away same of the ink. Capillary action maintains the capillary full of ink pulling in further ink from sources open to the capillary. In most writing pens the split in the nib serves as a capillary. Some recording pens have a fine cylindrical bore extending their length which serves as a capillary feed. Most fibrous and sponge-like materials carrying liquid will transfer it continuously to. a wettable surface due to inherent capillary characteristics provided by the many hairlike spaces in the material. In ball-point pens, the contacting portion or ball is revolved continuously through a reservoir so that the ink on the contacting surface is constantly renewed. In the present invention, capillary characteristics are avoided so that there is no continuous flow of ink Where an ink carrying member is drawn across the recording surface.

By some effect, not clearly understood, the presence of an electrostatic field will supply the force necessary for continuous ink flow. This force may properly be ascribed to coulomb attraction. In simple form, the invention is demonstrated by drawing a line with an inexpensive ink pen. When part of the nib is broken oil? destroying the capillary, the pen will no longer write. If the paper is then charged electrostatically to 300 or 400 volts and placed on a grounded substrate, the broken pen, grounded in common with the substrate, will write when drawn across the paper.

The present invention is useful also in duplicating processes. For example, using a zinc oxide binder plate as the photoreceptor, development with a wetting agent in accordance with the present invention permits the binder plate to be used in a duplicating process with water based ink. For this purpose, the zinc oxide binder plate should have a water repellent binder, such as many of the silicone resin binders that are conventionally used. The wetting agent developer then makes the image areas receptive to the water based ink, whereas the background areas would remain water repellent. The zinc oxide binder plate would then be used as an offset master in a single fluid duplicating process.

While the present invention has been described as carried out in specific embodiments thereof, there is no desire to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

We claim:

1. A method of developing a latent electrostatic image comprising:

(a) applying a homogeneous liquid having a bulk resistivity in the range of 5 X ohm-ems. to 10 ohm-cms. to the edge of a plain sheet metal blade; and,

(b) wiping the edge of said blade across said latent electrostatic image.

2. A method of developing a latent electrostatic image in accordance with claim 1 in which said homogeneous liquid is a solution of alkylphenyl polyglycol ether.

3. A method of developing a latent electrostatic image in accordance with claim 1 in which said homogeneous liquid contains a dissolved dye.

4. A method of image reproduction comprising:

(a) forming a latent electrostatic image;

(b) applying a homogeneous liquid wetting agent having a bulk resistivity in the range of 5x10 ohmcms. to 10 ohm-ems. to the edge of a nonporous blade;

(c) Wiping the edge of said blade across said latent electrostatic image at a rate of at least 2" per second to selectively wet said latent electrostatic image in the charged areas;

(d) transferring said wetting agent in image configuration to a water repellant surface of a transfer sheet; and,

(e) applying a water based ink to said transfer sheet so that it is selectively absorbed in the areas carrying said Wetting agent.

5. A method of image reproduction comprising:

(a) electrostatically charging a photoconductive insulating layer to in the range of 300 to 1000 volts;

(b) selectively discharging said layer by exposure to a light pattern of the image to be reproduced;

(c) applying a homogeneous liquid developer having a bulk resistivity in the range of 5 X 10 to 10 ohmcms. to a non-porous blade; and,

(d) drawing the edge of said blade across said layer While maintaining said blade at reference with respect to the voltage on said layer so that developer flows to said layer selectively in the charged areas.

6. A method of image reproduction according to claim 5 in which said blade is a fiat piece of sheet metal 2 to 10 mils thick.

7. In apparatus for developing a latent electrostatic image on an insulating layer with a liquid developer, the combination comprising:

(a) a smooth, nonporous applicator blade;

(b) means to wet said blade with said developer; and,

(c) means to provide relative contacting motion between said layer and blade so that said developer flows to the image areas on said layer under the force of coulomb attraction.

8. In Xerographic reproducing apparatus comprising: a rotatable xerographic drum; electrostatic charging means; exposure means; liquid developer means; transfer means and erasure means; the combination in said developer means comprising (a) a reservoir of liquid developer;

(b) a developer applicator having a plurality of axially radiating nonporous blades rotatable through said reservoir and against said drum; and,

(c) means to rotate said blades stepwise so that each blade maintains contact with said drum during a portion of the rotation of said drum before the next blade is stepped into position.

9. Xerographic reproducing apparatus according to claim 8 in which said means to rotate is actuated by a frame detector associated with said exposure means so that each of said plurality of blades remains in contact with said drum through one frame of exposure.

10. A method of image reproduction with simultaneous exposure and development comprising:

(a) applying a homogeneous liquid to a nonporous conductive blade;

(b) Wiping the edge of said blade across an electrophotographic plate;

(0) exposing said plate to a light pattern of the image to be reproduced; and,

(d) simultaneously with said wiping and said exposing, applying an electrical potential across said blade and said plate.

References Cited UNITED STATES PATENTS 3,079,272 2/1963 Grieg 961 3,096,198 7/1963 Schafi'ert 1.7 3,270,637 9/1966 Clark 961 3,290,251 12/1966 Nelson 25262.1 2,946,682 7/1960 Lauriello 96-1 2,947,625 8/ 1960 Bertelsen 96l 2,968,553 1/1961 Gundlach 96-4 3,052,213 9/ 1962 Schalfert 118637 3,059,614 10/1962 Limberger 118-637 2,605,684 8/1962 Nagels et al.

I. TRAVIS BROWN, Primary Examiner. NORMAN G. TORCHIN, Examiner. A. LIBERMAN, R. E. MARTIN, Assistant Examiners. 

